1. Field of the Invention
The invention relates to air-operated diaphragm brakes for vehicles and particularly to combination service and spring brake actuator assemblies.
2. State of the Prior Art
An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly which is actuated by means of an actuator assembly operated by the selective application of compressed air. Conventional air brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which causes actuation of the brakes when air pressure has been released. The emergency brake actuator includes a strong compression spring which forces application of the brake when air is released. This is often referred to as the spring brake.
Typically, the spring brake actuator is disposed in tandem with the service brake actuator. When full pressure is applied to the spring brake actuator, air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting the operation of the brake. When the brake is to be applied during normal driving operation, compressed air is provided to the service brake actuator which, acting against a diaphragm, causes a service brake push rod to be extended and causes the brakes to be applied with an application force which is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or an intentional exhaustion of air from the spring brake actuator, the brake will be mechanically activated by the force of the compression spring acting on the spring brake actuator rod which in turn acts upon the service brake push rod to apply the brakes. Thus, the spring brake portion serves both as a parking brake and an emergency brake.
In a typical prior art air brake system, the spring brake actuator and the service brake actuator are disposed in a single housing comprising a spring brake portion and a service brake portion. The service brake portion includes an air chamber partially defined by a flexible service diaphragm acting against a service brake push rod and a return spring to assure proper release of the brake when air is exhausted from the air chamber. The spring brake portion includes a spring chamber and an air chamber, both partially defined by a spring brake diaphragm acting against a spring pressure plate to compress the compression spring in the spring chamber when air pressure is applied to the spring brake diaphragm in the air chamber. An actuating rod extends through the spring brake diaphragm to integrally connect with the pressure plate. In operation, it is pushed outwardly from the air chamber through a housing opening and bearing provided with a pneumatic seal to engage the service diaphragm and push rod of the service brake, thereby causing the brake to be applied. The spring brake diaphragm is provided with a centrally disposed aperture having an annular edge and the actuator rod extends through the opening and engages the annular edge to form an airtight seal. The actuator rod is hollow with a central bore and a brake releasing caging bolt extends into the central bore. An end plate on the bolt engages the spring brake pressure plate to draw the spring to a compressed state when the spring is caged by rotation of the bolt.
The spring chamber on the spring side of the spring brake diaphragm is typically vented to the outside atmosphere to allow air to be exhausted from that portion of the housing when the diaphragm is actuated in the direction of the spring to compress the spring brake compression spring in the spring chamber. When the spring expands, the diaphragm is moved in a direction so as to enlarge the spring chamber and outside air is drawn into the spring chamber. Since the spring chamber is vented to the outside atmosphere under the truck chassis, the outside air drawn into the spring chamber is often accompanied by moisture and corrosive substances, such as road salt. Because the spring wears as it cycles and its bare metal becomes exposed as a result, the spring tends to corrode due to contaminants in the air. Also, because the spring brake compression spring must exert substantial force in order to act as an emergency brake, the spring is under great stress when it is compressed. The spring's durability is inversely proportional to the amount of corrosion and stress it experiences. In sealed housings, the spring cannot be removed from the housing without permanently deforming the housing. In other housings, the actuator must be replaced by a new actuator and the old actuator sent to a rebuilder. In any case, the actuator is not serviceable by the truck operator. Thus, when the spring brake compression spring breaks, the entire brake actuating unit must be replaced at substantial cost.
Attempts have been made to seal the spring chamber, including the provision of breather tubes to direct air from the spring chamber to other portions of the brake actuator and internal check valves to provide a one-way flow of air from the interior of the brake actuator to atmosphere. For example, U.S. Pat. Nos. 5,263,403 and 5,311,809 to Choinski et al. disclose a brake actuator having a spring brake chamber and a non-pressurized service brake chamber fluidly interconnected by a breather tube mounted to an external surface of the brake actuator. U.S. Pat. No. 4,890,540 to Mullins discloses a brake actuating unit having a housing provided with a pair of vent holes wherein one of the vent holes is plugged to prevent contaminants from entering the housing. Further, U.S. Pat. No. 3,896,706 to Newstead et al. discloses a brake unit having a tubular piston which includes a vent hole which extends into a spring housing which, in turn, is in communication with a filter at one end of the spring housing. The prior art solutions to the problem of contaminated air entering the spring chamber have proven to be unworkable or costly and complicated to manufacture.